1. Field of the Invention
The present invention relates to an optical scanning lens and an optical scanning apparatus, and more particularly to an optical scanning lens having an improved single-lens construction.
2. Description of Related Art
A conventional optical scanning apparatus includes an optical deflector which deflects at equiangular velocity a light flux that has been formed into a line image, which extends in a direction corresponding to a main scanning direction. The optical deflector has a deflective reflecting surface which is located in the vicinity of a position where the line image is formed. The deflective reflective surface condenses the light flux into a light spot on a surface to be scanned by an optical scanning lens so as to optically scan the surface to be scanned at a constant velocity.
This type of optical scanning apparatus has been widely used in optical printers and digital copying equipment. The phrase "the direction corresponding to the main scanning direction" used herein means the direction which corresponds to the main scanning direction on an optical path from a light source to a surface to be scanned, and the phrase "the direction corresponding to the sub-scanning direction" means the direction which corresponds to the sub-scanning direction on the above-described optical path.
The use of an optical scanning lens device having a single-lens construction has been proposed for such an optical scanning apparatus, for example, in Japanese Unexamined Patent Publication No. 7-174998. The purpose of using a single-lens construction is to achieve a compact size and lower assembly cost for the optical scanning apparatus.
It is desired that an optical scanning apparatus is sufficiently compact while also having an extensive effective main scanning width. For this reason, it is necessary to position an optical scanning lens as close to an optical deflector as possible in order to maximize the range of the effective deflection angle of an optical deflector while minimizing the overall size of the optical scanning lens and optical scanning apparatus.
In the device disclosed in Japanese Unexamined Patent Publication No. 8-248308 and No. 8-190062, however, as the optical scanning lens is positioned closer to the optical deflector, the image forming magnification of the optical scanning lens in the direction corresponding to the sub-scanning direction unavoidably increases. As a result, even the slightest displacement in the location where the line image is formed along the optical path caused by a slight positional error of the optical system components arranged for forming the line image in the direction of the optical axis causes a significantly displaced position of formation of a light spot resulting from the line image in the direction corresponding to the sub-scanning direction. This large position displacement of the light spot occurs because the longitudinal magnification increases with the square of the lateral magnification. Thus, the curvature of field in the sub-scanning direction would be significantly deteriorated and substantially less than the desired curvature of the original design.
Conversely, if an excessive emphasis is placed on reducing the image forming magnitude in the direction corresponding to the sub-scanning direction of the optical scanning lens as in the case of the device described in Japanese Unexamined Patent Publication No. 8-76011 or No. 8-297256, then the wave front aberration around the image formation point in the vicinity of a surface to be scanned would increase. As a result, it would be difficult to decrease the diameter of the light spot or the light spot would significantly increase in size if a cylinder lens or an optical scanning lens for forming the line image is inclined with respect to the optical axis.
In other words, according to the conventional devices described above, the cylinder lens and the optical scanning lens must be arranged and mounted with extremely high accuracy in order to obtain the desired effect since a slight optical dislocation leads to a serious deterioration in optical scanning performance.
Obviously, it is desirable that the optical scanning apparatus having only a single lens exhibits desired constant scanning velocity characteristics during the optical scanning process and ensures successful correction of curvature of field in both the main scanning direction and the sub-scanning direction so as to prevent the diameter of the light spot from being varied according to the image height of the light spot.
To achieve successful correction of the changes in the light spot diameter, it is not sufficient to only correct the curvature of field. In the device described, for example, in Japanese Unexamined Patent Publication No. 4-50908, although the curvature of field of the optical scanning lens in the sub-scanning direction may be corrected, the image forming magnification of the optical scanning lens in the direction corresponding to the sub-scanning direction varies according to deflection angle. As a result, the diameter of the light spot in the sub-scanning direction changes significantly according to the image height of the light spot.
Further, if a general polygon mirror is used as the optical deflector, then "sag" which is a spatial mismatch between the position where a foregoing line image is formed and a current line image is formed, may occur. The "sag" condition results from the deflective reflecting surface being arranged asymmetrically in relation to a zero deflection angle according to the deflection angle of a deflected light flux because the rotation axis of the polygon mirror is spaced at different distances from the deflective reflecting surfaces of the polygon mirror. This causes a problem in which the performance of the optical scanning lens deteriorates.
Thus, a single-lens construction of an optical scanning lens must satisfy many diverse conditions as described above. Therefore, when constructing the single-lens system, it is necessary to form at least one surface thereof into a special non-spherical surface. For this reason, fabrication using a plastic material is suited for making the optical scanning lens composed of a single lens. However, if the optical lens composed of the single lens is made as a biconvex lens, then the thickness tends to increase in a paraxial portion and the difference in thickness between the paraxial portion and a peripheral portion is consequently increased. As a result, when fabricating the optical scanning lens via a plastic molding process, known shaping errors such as a "sink mark" and "swelling" frequently occur during the molding process.
The above-described shaping errors cause serious deterioration in actual performance of the scanning lens and optical scanning apparatus no matter how good the design performance may be. This problem is especially significant when an optical scanning lens is disposed in the vicinity of an optical deflector to prevent an increase of the size of a scanning image forming lens in an effort to provide the scanning image forming lens with a wider angle while reducing the size of the optical scanning apparatus incorporating the lens and expanding the optical scanning range thereof.
When using the single-lens construction for the optical scanning lens, shaping the lens surface that is located closest to the optical deflector into a convex surface is effective for improving the constant velocity function represented by the f.theta. function. It should be noted, however, that when shaping the lens surface on the side closest to the optical deflector into the convex surface, the edge thickness of the lens decreases as the radius of curvature decreases and the difference in thickness between the paraxial portion and the peripheral portion also tends to increase, frequently leading to the problems described above. Furthermore, as the radius of the curvature of the lens surface on the side located closest to the optical deflector decreases, the incident angle of a deflected light flux relative to the lens surface, i.e. the angle formed by the outward normal line of the lens surface and the main beam of the incident deflected light flux, increases toward the periphery of the lens in the direction corresponding to the main scanning direction. This frequently causes a deteriorated curvature of field or non-constant velocity characteristics resulting from the shaping error of the lens surface.